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A thankful heart is not only the greatest virtue, but the parent of all the other virtues. [Lat., Gratus animus est una virtus non solum maxima, sed etiam mater virtutum onmium reliquarum.]

"Oratio Pro Cnæo Plancio", XXXIII, as quoted in Hoyt's New Cyclopedia Of Practical Quotations (1922), p. 336-37,
A thankful heart is not only the greatest virtue, but the parent of all the other virtues. [Lat., Gratus animus est una virtus non solum maxima, sed etiam mater virtutum onmium reliquarum.]